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History of Animals   

With regard to the crab, it has already been stated that it has
claws and feet, and their position has been set forth; furthermore,
for the most part they have the right claw bigger and stronger than
the left. It has also been stated' that in general the eyes of the
crab look sideways. Further, the trunk of the crab's body is single
and undivided, including its head and any other part it may possess.
Some crabs have eyes placed sideways on the upper part, immediately
under the back, and standing a long way apart, and some have their
eyes in the centre and close together, like the crabs of Heracleotis
and the so-called 'grannies'. The mouth lies underneath the eyes,
and inside it there are two teeth, as is the case with the crawfish,
only that in the crab the teeth are not rounded but long; and over the
teeth are two lids, and in betwixt them are structures such as the
crawfish has besides its teeth. The crab takes in water near by the
mouth, using the lids as a check to the inflow, and discharges the
water by two passages above the mouth, closing by means of the lids
the way by which it entered; and the two passage-ways are underneath
the eyes. When it has taken in water it closes its mouth by means of
both lids, and ejects the water in the way above described. Next after
the teeth comes the oesophagus, very short, so short in fact that
the stomach seems to come straightway after the mouth. Next after
the oesophagus comes the stomach, two-horned, to the centre of which
is attached a simple and delicate gut; and the gut terminates
outwards, at the operculum, as has been previously stated. (The crab
has the parts in between the lids in the neighbourhood of the teeth
similar to the same parts in the crawfish.) Inside the trunk is a
sallow juice and some few little bodies, long and white, and others
spotted red. The male differs from the female in size and breadth, and
in respect of the ventral flap; for this is larger in the female
than in the male, and stands out further from the trunk, and is more
hairy (as is the case also with the female in the crawfish).
So much, then, for the organs of the malacostraca or crustacea.

With the ostracoderma, or testaceans, such as the land-snails
and the sea-snails, and all the 'oysters' so-called, and also with the
sea-urchin genus, the fleshy part, in such as have flesh, is similarly
situated to the fleshy part in the crustaceans; in other words, it
is inside the animal, and the shell is outside, and there is no hard
substance in the interior. As compared with one another the testaceans
present many diversities both in regard to their shells and to the
flesh within. Some of them have no flesh at all, as the sea-urchin;
others have flesh, but it is inside and wholly hidden, except the
head, as in the land-snails, and the so-called cocalia, and, among
pelagic animals, in the purple murex, the ceryx or trumpet-shell,
the sea-snail, and the spiral-shaped testaceans in general. Of the
rest, some are bivalved and some univalved; and by 'bivalves' I mean
such as are enclosed within two shells, and by 'univalved' such as are
enclosed within a single shell, and in these last the fleshy part is
exposed, as in the case of the limpet. Of the bivalves, some can
open out, like the scallop and the mussel; for all such shells are
grown together on one side and are separate on the other, so as to
open and shut. Other bivalves are closed on both sides alike, like the
solen or razor-fish. Some testaceans there are, that are entirely
enveloped in shell and expose no portion of their flesh outside, as
the tethya or ascidians.
Again, in regard to the shells themselves, the testaceans
present differences when compared with one another. Some are
smooth-shelled, like the solen, the mussel, and some clams, viz. those
that are nicknamed 'milkshells', while others are rough-shelled,
such as the pool-oyster or edible oyster, the pinna, and certain
species of cockles, and the trumpet shells; and of these some are
ribbed, such as the scallop and a certain kind of clam or cockle,
and some are devoid of ribs, as the pinna and another species of clam.

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